During a press conference on Wednesday night, Metropolitan police assistant commissioner Mark Rowley explained that authorities believe Masood was inspired by Islamic extremism. At the time, Rowley declined to name the suspect, however, saying that it was an active investigation.

Masood was not the subject of any active investigations, police said, but in the past he had been convicted for assault and possession of offensive weapons. His first conviction in 1983 was for criminal damage. His most recent was in December 2003 for possession of a knife.

On Thursday, the self-described Islamic State issued a claim of responsibility for the attack through its Amaq news agency. It is unclear whether the group had any direct involvement.

Some early media reports on Wednesday misidentified the attacker, mistakenly stating that he was a radical Islamic preacher who was well known to authorities. However, the man turned out to be in prison, serving a sentence for violating the U.K.’s Terrorism Act after being arrested last year in Hungary for illegally leaving the country.

Three other officers were injured in the attack when Masood reportedly mowed down citizens on Westminster Bridge with a vehicle.

A doctor at St. Thomas Hospital described some of the victims’ injuries as “catastrophic,” according to the Press Association. At King’s College hospital, at least two victims remained in critical condition on Thursday.

We are still treating two patients who are in a critical condition and four who are stable. #Westminster [2/2]

Vehicle attacks have become a common terror tactic in Europe in recent years. Trucks driven into crowds in France and Germany killed a total of nearly 100 people in 2016, and ISIS has repeatedly urged its supporters to use ramming attacks as a means of killing civilians.

The incident at Westminster was also similar to an attack in the Canadian capital of Ottawa. In October 2015, Islamic extremist Michael Zehaf-Bibeau shot and killed an army corporal outside Parliament before attempting to storm the building.